Finally in our component tour, we come to crystals. Just as their name suggests, they are a small block of quartz (silicon dioxide). Quartz crystal is a type of material known as a piezoelectric . This is a substance that generates a voltage when it is stressed (compressed, stretched, twisted). This effect is utilized in microphones. The sound vibrates the piezoelectric material, and it produces a small AC voltage that is directly proportional to the original sound that created it. This voltage is then amplified for broadcasting, recording, or processing.

The opposite effect is also true for piezoelectric materials. Apply a voltage and the piezoelectric material will contort or vibrate. Some speakers use piezoelectric materials to produce their sound. However, most use other techniques, such as electromagnetics. Most loud buzzers are based on piezoelectrics.

Now, the neat thing about quartz is that for a block of a given size, it will vibrate at a given (and fixed) frequency. For that reason, it can be used as an oscillator to generate a sine wave, which in turn can be used to generate timing signals for microprocessors and other digital circuits. Just about every computer system will have a crystal (or two) somewhere on its circuit board, generating the timing that ultimately drives the whole machine. That crystal is simply a small block of quartz, plated at either end with wires attached and encased in a metal can.

The schematic symbol for a crystal is shown in Figure ...

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